The current mean protein requirement for adults of all ages is 0.6 g protein.kg-1.d-1, with a suggested safe protein intake set a 0.75 g protein.kg-1.d-1 in 1985 by the Joint FAO/WHO/UNU Expert Consultation (WHO) and at 0.8 g protein.kg-1.d-1 (the Recommended Dietary Allowance, RDA) in 1989 by the US National Research Council (NRC). Nitrogen balance (NBal) is the current standard, and only accepted, method used to establish total protein (nitrogen) requirements and allowances. Both the WHO and NRC indicated that insufficient data were available from studies done with elderly subjects to determine a protein requirement with confidence. Thus, the current recommendations for elderly people are largely extrapolations from NBal studies done in young men. Yet, sarcopenia (the age-related loss of muscle mass), and changes in physical activity, food intake, and the frequency of disease in elderly people may contribute to an altered protein requirement in this population. Limited NBal data from our laboratory suggest that the protein requirement of elderly people is actually 50% higher than the accepted requirement. Other researchers using 1-13C-leucine balance studies, recently suggested that the protein requirement of elderly people is similar to or less than that of younger adults. We have shown that adherence to a low-protein diet by elderly women for 10 weeks adversely effected fat-free mass, muscle mass, and muscle strength and function. Our preliminary data now indicates that elderly people who consumed eucaloric diets providing the RDA for protein for 14 weeks lost fat-free mass and muscle mass, and gained body fat. The public health implications of this accommodation to the protein RDA are largely unknown. This research will, for the first time, test whether the mean protein requirement of 7-85 year-old people is greater than that of 18-30 year- old people. Women and men will be studied separately. A double-blinded NBal study design will be used during four separate 7 day NBal trials. Strict dietary control will be maintained by giving the subjects energy- balanced menus with either 0.40, 0.60, 0.80 or 1.00 g protein.kg-1.d- 1.1-13C- leucine balance studies will be done at each protein intake to assess age related differences in leucine oxidation, utilization oxidation, utilization, incorporation into albumin and the apparent protein requirement. Clinical markers of visceral protein status will also be measured. The results of the study will contribute to our understanding of the metabolism and requirement for protein, and will be critical to establishing a RDA for protein of elderly people.